The present invention relates generally to pumps and, more particularly, to wear resistant housings for industrial screw pumps used in abrasive environments and method of manufacturing the housings.
In a conventional rotary axial-screw pump, pressure is developed from the inlet or suction port to the outlet or discharge port of the pump in near-even stage-to-stage increments. The stages are defined in accordance with the number of moving-thread closures or isolated volumes formed by meshing of the pump rotors in close tolerance with the pump walls between inlet and outlet ends of the pump.
At a given size and flow rate, pump pressure is developed along the moving thread closures as liquid progresses through the pump. The number of closures is usually proportioned to the desired level of delivered outlet pressure, i.e., the greater the pressure, the greater the number of closures necessary. These closures result in the pump developing an internal gradient of progressively stepped pressure increments. Properly applied, a rotary axial-screw pump will pump a wide range of liquids, from a high-viscosity oil to a light water-soluble oil in solution.
In practice, rotary axial-screw pumps are often used to pump contaminated or recirculated fluids which contain fine metal, grit, sand and/or other abrasives. The ductile, machinable metals typically used internally in these pumps and under these conditions, however, often experience considerable wear.
Conventional solutions to this problem have been limited. For example, to maintain screw pump efficiency, the design of pump rotor housings dictates that tri-bore areas, i.e., the bored areas of the rotor housings where the rotors are located, have very smooth finishes and be made to close tolerance with the rotors. The need for a close tolerance has also precluded designers from opening-up pump clearances to allow for the free passage of contaminated fluid.
Use of conventional sprayed hard coatings have also been found unsuitable. This is due, at least in part, to the difficulty in applying them evenly, as most hard coating spray nozzles do not fit easily into conventional small tri-bore diameters. Cast or dipped hard coatings have also been difficult to apply evenly since tri-bore diameters are relatively small as compared to their length. Paint-on coatings, on the other hand, often cannot withstand the pressure and velocity associated with occasional contact between the rotors and housing, causing them to crack and peel during pump operation.
While solid ceramic rotor housings have been considered relatively effective, the expense of the necessary materials combined with the time and difficulty of their production has made them less attractive.
Another prior method of manufacturing wear resistant screw pump housing is disclosed in German Offenlegungsschrift Patent, DE4016841A1 laid open to public inspection on Nov. 28, 1991. In this German patent, in order to avoid the difficulties which occur in the previous methods of manufacture by means of boring and broaching overlapping bores and conditioning of the housing during the manufacture, the screw pump housing was centrally divided in order to enable the internal housing surface to be operated on by external means such as profile milling or profile grinding.
While dividing the housing into two symmetrical components solves some of the problems associated with the expense and difficulty of providing wear resistant material on the internal surfaces of the housing, some problems remained unsolved regarding the symmetry of the bore when the two components were reassembled into the housing.
Thus, there is a need for improved methods of manufacturing wear resistant housings for industrial screw pumps used in abrasive environments that are economical to manufacture, and which results in the closer tolerances of the opposing internal surfaces of the housing.